1. Field of the Invention
This invention relates to a process for the oxidation treatment of coal, particularly low-grade coal (or low-quality coal) having a high moisture content, and an apparatus therefor.
2. Description of the Related Art
Low grade coal such as brown coal and subbituminous coal are found in abundance, but they have a high moisture content. Accordingly, their calorific value per unit weight is low, their transportation is uneconomical, and their reactivity is so high that they are liable to spontaneous combustion during storage or transportation. Owing to these problems, they are not positively utilized at present. In order to utilize such low-grade coal effectively, a variety of reforming processes have been proposed.
As a process for reforming coal having a high moisture content by dehydration and heating, there has been proposed a coal reforming process wherein coal is loaded on a rotatable annular moving grate (hereinafter referred to as xe2x80x9ccircular gratexe2x80x9d), heated by supplying the coal bed with a hot gas having an oxygen concentration of not greater than 5%, cooled to a temperature lower than its ignition point by passing a cooling gas having an oxygen concentration of not greater than 5% through the coal bed, and then discharged (Japanese Patent Publication No. 5-66434). This patent discloses an exemplary coal reforming apparatus in which its internal space is partitioned into zones and different heat treatments such as drying, carbonization and cooling are carried out in the zones. However, no mentioned is made of the use of this apparatus for carrying out an oxidation treatment.
When low-grade coal is reformed by a dehydration and heating treatment, it exhibits high activity (i.e., spontaneous combustibility). In order to prevent spontaneous combustion of such reformed coal, it is usually subjected to an oxidation treatment. Generally, this process for reforming low-grade coal comprises the steps of subjecting low-grade coal to a dehydration and heating treatment so as to remove the moisture contained therein and thus increase its calorific value per unit weight, subjecting the coal to an oxidation treatment for the purpose of minimizing its spontaneous combustibility, and subsequently cooling the coal to a temperature lower than its ignition point.
For example, a process for the oxidation treatment of coal by using an apparatus illustrated in FIG. 3 has been proposed in the prior art (Japanese Patent Publication No. 4-29715). Specifically, coal is fed to the upper part of the tank through a line 66 and allowed to fall by gravity, while air is supplied to air distributors 70 having air injection nozzles through a line 68, and injected into the lower part of the tank from an air outlet. Thus, the oxidation treatment of coal is carried out by bringing the coal into counterflow contact with air while regulating the residence time of the coal in the tank.
However, in the conventional process wherein coal is subjected to an oxidation treatment in a fluidized state as illustrated in FIG. 3, the coal is pulverized owing to the collision of coal pieces and the collision of coal pieces with the wall surfaces and internal parts of the tank, so that a large amount of coal dust is produced. This coal dust poses a problem because it fouls the apparatus by adhering to its internal surfaces and causes a clogging in the apparatus and piping. Moreover, the coal dust not only imposes a considerable load on attachment devices such as cyclone, but also reduces the yield of the product.
An object of the present invention is to provide a coal reforming process wherein an oxidation treatment is carried out without producing coal dust, and a coal reforming apparatus therefor.
In a coal reforming process comprising a series of steps including the aforesaid dehydration and heating treatment step and the aforesaid oxidation treatment step, the present invention relates to an improved oxidation treatment method for reducing the spontaneous combustibility of coal and an apparatus therefor.
That is, the present invention provides a coal reforming process wherein, as one of the coal reforming treatment steps, an oxidation treatment step is carried out on a circular grate.
In the process of the present invention, the circular grate may be separated into a plurality of zones, and the reaction rate may be controlled by independently regulating the temperature, flow rate and oxygen concentration of a gas passed through each of the separated zones.
Moreover, in the practice of the present invention, there may be used a coal reforming apparatus comprising a circular grate, the circular grate being separated into a plurality of zones which include fixed bed zones and mixing zones for fluidizing the coal properly between adjacent fixed bed zones.
This coal reforming apparatus may further comprise means for independently regulating the temperature, flow rate and oxygen concentration of a gas passed through each of the separated zones.
Furthermore, in the coal reforming process of the present invention, there may be used a rotatable circular grate equipped with an enclosed space having a plurality of zones. In this case, the oxidation treatment of coal with a heated gas is carried out in a first zone, and the coal is uniformly dispersed and mixed in a second zone.
In a preferred embodiment, the aforesaid circular grate is equipped with two or more enclosed spaces, and the aforesaid oxidation treatment step and the aforesaid dispersing and mixing step are repeated two or more times.
The aforesaid heated gas may be supplied either from below the coal bed or from above the coal bed.
According to the present invention, an efficient oxidation treatment step can be carried out in a coal reforming process, without causing powdering or temperature variation during heating. Moreover, heat generation can be controlled by defining a plurality of oxidation treatment zones and supplying them with heated gases having different temperatures, flow rates and oxygen concentrations.
The present invention can produce the following effects.
(1) As contrasted with the prior art, most of the oxidation treatment step for coal is carried out in a fixed bed (i.e., a non-fluidized bed). This can minimize powdering of coal due to the collision of coal pieces or the collision of coal pieces with the internal surfaces of the apparatus, and thereby prevent a reduction in the yield of the product.
(2) The form of a coal bed may comprise a combination of a fixed bed and a fluidized bed, so that the coal treated in a fixed bed can be fluidized and mixed properly. This can make uniform the temperature distribution occurring chiefly in the thickness direction of the coal bed and thereby prevent the formation of hot spots.
(3) It is possible to define a plurality of oxidation treatment zones and supply them with heated gases having different temperatures, flow rates and oxygen concentrations. This can accelerate the oxidation reaction and thereby reduce the time required for the oxidation treatment, while controlling heat generation due to the adsorption of oxygen to coal.
(4) The feed rate of a heated gas may be increased by supplying it from not only bellow the coal bed but also above the bed, so that the heat generated by the oxidation of coal can be removed rapidly. This can increase the thickness of the coal bed and thereby improve the throughput of the apparatus or make the apparatus more compact.